Model's Limbs Shortening/Flattening When Bent
Select your armature in Object mode, as you can see it has been scaled and therefore it stretches your mesh as soon as you move bones, so just apply the scale with a CtrlA and it will work fine.

As for your hair rigging, your hair is controlled by bones that are not supposed to control it, you need to assign it to the head vertex group:
Go in Edit mode, select the hair mesh (press L to automatically select the whole mesh):


Go in the Properties panel &gt; Object Data &gt; Vertex Groups, in the dropdown menu on the right of the vertex groups list, click on Remove From All Groups:



Select the group called Spine006 (it's the group that is controlled by your head bone, when you parent With Automatic Weight, vertex groups are automatically created with the name of the bone that is controlling them) and click on the Assign button: